The present invention relates to adjusting a transmit power spectrum of a subscriber device of a communication network by a transmit/receive means of the communication network, as it is for example required in DSL technology (DSL=digital subscriber line).
Due to the plurality of pairs of wires in a telephone cable from a central office (CO) to connected subscribers or subscriber devices, respectively, crosstalk occurs despite a direct current insulation of the pairs of wires, both near-end crosstalk (NEXT) and also far-end crosstalk (FEXT). The same result from capacitive or inductive couplings. By twisting the individual pairs of wires of a cable, this crosstalk is minimized so far that it may be neglected in the voice band. In data transmission technology, like e.g. DSL technology, however, apart from the voice band further frequency ranges are used, in which this crosstalk has a very strong effect on the possible range and transmission speed.
Near-end crosstalk means, that a transmitter on the “near side” of a receive device, i.e. both receive device and also transmitter are part of different subscriber terminal devices or both receive device and also transmitter are part of the central office, overcouples transmit signal portions into the receive branch of the receive device.
In contrast to that, far-end crosstalk means, that, in addition to the receive signal determined for the same, sent from a far-end first transmitter, i.e. first transmitter and receiver are on different sides of subscriber and central office side, a receiver receives portions of a second transmit signal sent from a far-end second transmitter, i.e. also second transmitter and receiver are on different sides.
In an xDSL network, wherein “x” stands for different implementations of DSL technology, the subscriber devices (CPE=customer premises equipment) are typically in a different distance from the central office (CO). A so called near-far problem results from this. If all subscriber devices of the network transmitted with the same transmit power spectral density (TX-PSD, transmit power spectral density), the CPEs closer to the CO would respectively couple a high amount of crosstalk into the lines of the CPE further away from the CO and thus cause differently strong interference power spectral densities of the individual subscribers.
For this reason, within the scope of VDSL standardization (VDSL=very high speed digital subscriber line), a so called upstream power back-off (UPBO) is defined. This is a method for determining the transmit power spectral densities (TX-PSDs) of subscribers on pairs of wires of different lengths in a multiple-pair communication table, such that the interference power spectral density caused by each pair of wires comprises the same value due to far-end crosstalk (FEXT-PSD) at the CO.
For performing the UPBO method as a standard the steps discussed in the following are performed during a training phase during a connection setup between CO and CPE. At the beginning of a training phase, the CO transfers standard signals to be presented to the CPE. The same may for example contain information about a signal power with which signals were transmitted from the CO, default values with regard to a power with which the upstream signals sent from the CPE are to arrive at the CO, and also shaping coefficients (Ai, Bi) for frequency bands i for a frequency-dependent shaping of the spectral transmit power density of the CPE. Using these parameters, in a first step by the CPE power parameters of the physical connecting passage between CO and CPE are determined. Here, for example, at the CPE a receive signal strength is determined and using the information about the original central-side transmit power of the signals from the CPE a measure kl0 for the attenuation is estimated, which the signals experience on the way from the CO to the CPE. Using this knowledge, on the subscriber side a transmit power may be determined such that the signals reach the CO as accurately as possible with the power requested from the CO. Values for this requested receive power or receive power spectral density, respectively (RX-PSD), may for example be given by a network operator for example via network management adjustments. In a further step of the training phase, the attenuation ratio is transmitted from the CPE to the CO.
The indicated standards thus define a UPBO method which approximates the request for a respectively equal interference power density of the pairs of wires at the CO by realizing the respectively equal receive power spectral density. By this approximation, subscribers on shorter lines cause a lower FEXT-PSD than subscribers on longer lines. This disadvantage could be prevented by a frequency-dependent correction of the TX-PSD on the subscriber side.